Problem copying mail folder to local mail server

I am running OS/X Server on my local iMac and enabled the Mail service. I am trying to copy local mail folders from the Mail app up to the server. I do this by dragging them to a folder on the server I created called "Archive". For most of them, everything works fine. But for some of the folders I get the following error:

Some items could not be copied to "Archive"
Failed to open ""

This is repeatable and it creates an empty folder of the name I am trying to copy. If I delete the folder and try again, I get the same error.

The folders I am trying to copy were originally imported from .mbox files that I exported from Outlook for Mac. I am trying to move away from Outlook and would like to have all of my mail stored on a local server so it is accessible from both my desktop and notebook.

Any ideas?

Thanks,

GPM